Agricultural machines such as tractors, construction machines loaders or other types of operating machines, typically have a hydraulic system with which one or more hydraulic consumers such as hydraulic cylinders, hydraulic motors or other hydraulically driven components. Such hydraulic systems include hydraulic pumps that can be connected directly or over a connecting gearbox with fast or slow fixed gear ratios to the drive shaft of a drive engine. Thereby the maximum volume flow that can be conveyed by the hydraulic pump varies with the rotational speed of the drive engine. The faster the rotational speed of the drive engine the larger is the volume flow that can be conveyed by the hydraulic pump. In adjustable load-sensed controlled hydraulic pumps, so called adjustable pumps, as they are being applied today in the state of the art, the maximum volume flow conveyed can be made to conform to the demand of the hydraulic consumers. This is performed usually over a so-called conveyed volume-flow-controller that controls or maintains a predetermined control pressure difference between the pressure of the outlet of the adjustable pump and the load sensing signal (in the following called LS-signal). The conveyed volume flow controller of a LS-controlled adjustable pump now operates in such a way that it adjusts the conveyed volume flow of the adjustable pump in such a way that the predetermined control pressure difference, that can be adjusted on the conveyed volume flow controller can be provided as an input by means of an adjusting spring and is maintained constant at all times. The exact method of operation of such a pressure-based volume flow controller can be reviewed in the relevant literature and as such is the state of the art.
The conveyed volume flow that can be delivered by a hydraulic valve to a hydraulic consumer depends directly upon this control pressure difference. A certain control pressure is adjusted by means of the adjusting spring and an adjusting piston of the conveyed volume flow controller in that it forces the adjustable pump to maintain a control pressure difference corresponding to this adjusted pressure between the outlet of the adjustable pump and the consumer (L-S Signal). In order to attain this control pressure difference, it pivots the conveyed volume flow control adjusting unit upward in order to begin to convey a corresponding conveyed volume flow that can be controlled or adjusted as a function of the adjusting piston. Here the adjusting piston is connected hydraulically with the conveyed volume flow controller and changes its position as a function of the control pressure difference existing or provided as input at the conveyed volume flow controller. The conveyed volume flow control unit may for example include a pivoting disk that is connected with a control or lifting piston where the rotating movement of the pivoting disk is converted into a linear movement of the lifting piston. The conveyed volume flow conveyed by the adjustable pump flows through the lines and the valves of the hydraulic system and thereby generates certain pressure losses in the lines and each of the valves leading to the consumer. The pressure that then develops behind the valves or at the consumer is reported back as load pressure (LS-signal) to the adjusting pump (over a load sensing line (L-S-line) that is connected to the conveyed flow controller) and impels the adjustable pump to convey such an amount of flow that the pressure at the outlet of the adjustable pump is higher by the control pressure difference than the pressure at the consumer delivered by the L-S-Signal.
The further a valve now is from the adjustable pump, the higher are the pressure losses occasioned by the longer lines, which leads to the effect, that valves that are located further away from the adjustable pump than other valves, permit less volume flow to reach the consumer, although those valves are configured identically. In order to compensate for this effect, a known practice is to apply valves that transmit an increased load signal to the pump, as is disclosed for example in EP 176 0 325 A2.
Accordingly a simplified conclusion is that a certain pressure is required to force a certain volume flow through a line and/or a valve. Since the pressure losses increase during the flow through the lines or valves, it would therefore be advantageous to maintain the cross sections and the conduct of the lines and the bores as large as possible, as well as keeping the losses over the valves as low as possible by the configuration if a certain volume of hydraulic fluid is to be provided to a consumer. If the losses become too great and thereby the volume flow is reduced, this can be compensated for by increasing the cross section of the valve openings, in other words, by changing the cross section at the valve openings, volume flows can be changed, that is, they can be increased or decreased.
Other possibilities of changing the volume flow lead to a change in the adjusting force provided at the conveyed volume flow controller of the adjustable pump. In that way, EP 0 439 621 B1 discloses that for a precision operation of the hydraulic system, the control pressure difference at the adjustable pump can be reduced by manual operation of an adjustable force at the conveyed volume flow controller, which results in a lower maximum volume flow in the hydraulic system or in the valves.
Now the problem is that is that it may be advantageous for environmental or economical reasons to operate a hydraulic system of an operating machine in the lower range of rotational speeds. This has the result that with today's adjustable pump sizes too little volume flow is available for the applications, which leads to the application of larger adjustable pumps, so that large volume flows can be conveyed at low engine rotational speeds. This results in turn in very large volume (not utilized) flows being conveyed at high engine rotational speeds that lead to very large power losses in the overall power balance. These problems could be overcome at least partially, by raising the control pressure difference of the pump, that finally would lead to a higher fuel consumption of the machine, since a certain power output is required, or a certain conveyed volume flow is required, in order to attain the necessary control pressure difference. Moreover the possibility exists of designing all lines and valves for the maximum pump power output that would in turn lead to very high cost for the individual components and to space problems on the operating machine. EP 349 092 B1 discloses a further possibility, to permit possible high volume flows at low engine rotational speeds, but to limit the volume flow at high engine or rotational speeds. Here the maximum conveyed volume flow of the adjustable pump is limited, where the conveyed volume flow amount of the adjustable pump is measured or monitored (for example, by a measurement of the position of the conveyed volume flow control mechanism, for example the adjustment angle of an adjusting disk or a pivot disk). These adjustable pumps and the corresponding electronic control systems however are costly and expensive.